In recent times, brake systems of the brake-by-wire type have been developed for motor vehicles and especially for passenger cars. Such a brake system is, for example, disclosed in German patent publication 4,022,671. This brake system includes a brake-value transducer which is connected to two central modules which, in turn, are connected via a data bus to two wheel modules. The central modules and the wheel modules assigned thereto can exchange data in serial operation via the data busses.
A braking operation can be carried out with the aid of the brake system disclosed in German patent publication 4,022,671 as described below. The brake value inputted by the driver is conducted via the brake-value transducer to the central modules and is here first converted into individual brake values for the individual wheel modules. Thereafter, the individual brake values are conducted from the central modules to the wheel modules via the data busses and, on the basis of the individual brake values, the braking operation at each wheel of the vehicle is initiated by the wheel modules.
Different loads on the wheels can occur, for example, because of driving through a curve and/or because of load. By generating individual brake values, these different wheel loads or the different temperatures of the brakes can be considered so that an overheated brake can be detected and controlled out and yaw moments of the motor vehicle can be compensated. However, data from the individual wheel modules must be transmitted to the central units via the data bus in order to consider the operating parameters of individual wheel brakes. As a consequence thereof, the data busses are "in use" from time to time and no data can be transmitted from the central modules to the wheel modules for initiating a braking operation. This then leads to an unwanted delay of the braking operation.
Furthermore, it is not always guaranteed that the individual brake values, which are transmitted by the central modules, always arrive at the same time at all four wheel modules so that the staggered arrival of the brake values can cause an unwanted pulling of the motor vehicle to one side or the other. Finally, the brake system disclosed in German patent publication 4,022,671 can be characterized as unsafe in that a complete disabling of the brake system can occur in the event that the central module becomes inoperative because, in this case, the pregiven brake value is no longer registered by the central modules and the individual brake values are no longer transmitted from the central modules to the individual wheel modules.
European patent publication 0,439,559 discloses a system having distributed modules which communicate with each other via a serial data bus. With this system, commands can be executed in individual modules without a time delay. For this purpose, commands are transmitted from a central module to other modules via the data bus at any desired time point and the commands are stored in these other modules. The command for executing the stored command is supplied to the modules via separate data lines and the stored command is executed directly after the command is received.
The system disclosed in European patent publication 0,439,559 is, however, not suitable for use in brake systems of the brake-by-wire type because the command generation takes place automatically in the central module as does the command transmission between the central module and the other modules and the storage of the fixed commands takes place in the modules and cannot be influenced by the driver of the motor vehicle. However, a braking operation is precisely such an operation which is intended to be influenced by the driver via a suitable actuation of the brake pedal and is dependent upon a particular situation.